Method and device for detecting the detachment of the venous needle from a patient during dialysis

ABSTRACT

A method of detecting the detachment of the venous needle from a patient during an extracorporeal blood treatment in a dialysis machine having an extracorporeal blood circuit provided with an arterial branch and a venous branch. The arterial pressure (Pa) is measured in the arterial branch and the venous pressure (Pv) is measured in the venous branch, and the determination is made of whether or not decreases of the arterial pressures (Pa) and venous pressures (Pv) are occurring during the dialysis treatment in normal conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/IB02/02354, filed Jun. 24, 2002, the content of which isincorporated herein by reference, and claims the priority of ItalianPatent Application MI2001A001395, filed Jun. 29, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of detecting the detachment ofthe venous needle from a patient during an extracorporeal bloodtreatment in a dialysis machine.

2. Description of the Related Art

As is known, blood consists of a liquid component called the bloodplasma and a corpuscular component formed by the blood cells, includingthe red corpuscles among other components. In renal insufficiency, theblood has, in addition to the aforesaid components, particles of lowmolecular weight (referred to below as solute) which have to beeliminated by a dialysis treatment carried out with a dialysis machine.

A dialysis machine of the known type generally comprises anextracorporeal blood circuit, a dialysate circuit and a filter, which islocated in the aforesaid circuits and comprises a blood compartment anda dialysate compartment, which are separated from each other by asemi-permeable membrane, and through which pass, respectively, the bloodto be treated and the dialysate, generally flowing in counter-currentmode.

During the dialysis treatment, the undesired particles contained in theblood migrate from the blood compartment to the dialysate compartmentthrough the semi-permeable membrane both by diffusion and by convection,as a result of the passage of some of the liquid contained in the bloodtowards the dialysate compartment. Thus the patient will have lost someweight by the end of the dialysis process.

The extracorporeal circuit is connected to the patient by means of anarterial needle and a venous needle, which are inserted into fistulasformed in the patient's cardiovascular system, so that they can,respectively, collect the blood to be treated and return the treatedblood to the patient's cardiovascular system. The extracorporeal circuitcomprises a peristaltic pump and a dropper located in the arterialbranch and in the venous branch respectively. The detachment of one ofthe aforesaid needles from the fistula causes an interruption of theaccess to the patient's cardiovascular system. The detachment of thevenous needle, if not detected in good time, has particularly seriousconsequences, because it can cause a significant blood loss in thepatient. Various attempts have therefore been made to provide methodsfor detecting the detachment of the aforesaid needles, and particularlyof the venous needle.

One of the aforesaid known methods is described in WO 99/12588. Thismethod is based on the electrical conductivity of the blood, andconsists of the injection of a current into a closed circuit consistingof the extracorporeal circuit and the patient's cardiovascular system,and the measurement, by means of a measuring instrument located in theaforesaid extracorporeal circuit, the current variations which arecaused by the detachment of one or both of the needles. For this method,the current injection and the measurement of the current variation haveto be carried out by inductive coupling, in other words by means ofwindings located at specified points along the extracorporeal bloodcircuit.

The method described above has various drawbacks. In particular, thismethod, although theoretically valid, cannot provide satisfactoryresults from the practical point of view, since the high electricalimpedance produced by the peristaltic pump, which effectively interruptsthe continuity of the blood flow, makes it necessary to operate withrelatively high currents in order to make use of the low conductivity ofthe materials, generally PVC, which are used to form the extracorporealcircuit, the filter, the peristaltic pump and the dropper. The use ofrelatively high currents is most inadvisable in a machine connected to apatient, and, even if these currents could be used, it would not bepossible to transmit them by means of an inductive coupling, which,among other considerations, generates additional parasitic currentswhich interfere with measurements. In some dialysis machines, thedropper also creates a high impedance, of the same order of magnitude asthat created by the peristaltic pump, and thus exacerbates one of theproblems described above.

Consequently, because there is a requirement to operate with relativelylow currents, and because the impedance of the peristaltic pump, and ofthe dropper in most cases, is high, the detachment of one of the needlescauses current variations which are not easily identifiable and whichcan be confused with the background noise of the measuring instrument.

Furthermore, this method does not allow for the fact that the patientmay be connected to earth and that the filter itself is necessarilyconnected to earth, since the dialysate circuit is connected to earth tomeet the requirements of the safety regulations for dialysis machines.Therefore the electrical circuit assumed in the aforesaid applicationdoes not truly represent the real analogy of a dialysis machine inelectrical terms.

It is also known from U.S. Pat. No. 6,221,040 a method for monitoringvascular access using pressure signals in a way different from thepresent invention.

The object of the present invention is to provide a method of detectingthe detachment of the venous needle from a patient during anextracorporeal blood treatment in a dialysis machine, which overcomesthe drawbacks of the known art.

SUMMARY OF THE INVENTION

According to the present invention, a method is provided for detectingthe detachment of the venous needle from a patient during anextracorporeal blood treatment in a dialysis machine having anextracorporeal blood circuit comprising an arterial branch and a venousbranch, the method comprising the steps of measuring the arterialpressure in the arterial branch and the venous pressure in the venousbranch, and determining whether or not essentially simultaneousdecreases of the arterial and venous pressures are occurring during thedialysis treatment in normal conditions.

The present invention also relates to a device for detecting thedetachment of the venous needle from a patient during an extracorporealblood treatment in a dialysis machine.

According to the present invention, a device is provided for detectingthe detachment of the venous needle from a patient during anextracorporeal blood treatment in a dialysis machine having anextracorporeal blood circuit comprising an arterial branch and a venousbranch, the device being characterized in that it comprises pressuredetectors for measuring the arterial pressure in the arterial branch andthe venous pressure in the venous branch, and a control unit fordetermining whether or not decreases of the arterial and venouspressures are occurring during the dialysis treatment in normalconditions.

Note that the control unit evaluates preferably if said decreases of thevenous and arterial pressures are occurring in a time interval, which isrelatively short (some seconds).

BRIEF DESCRIPTION OF THE DRAWINGS

To enable the present invention to be more clearly understood, apreferred embodiment of it will now be described, purely by way ofexample and without restrictive intent, with reference to the attachedfigures, of which

FIG. 1 is a schematic view, with parts removed for clarity, of adialysis machine fitted with a device for detecting the detachment ofthe venous needle, constructed according to the present invention;

FIG. 2 is a graph showing the blood flow, recorded in the extracorporealblood circuit in a dialysis machine during an experimental test whichsimulates a dialysis treatment with detachment of the venous needle; and

FIGS. 3 and 4 are graphs showing the venous pressure and arterialpressure measured in the extracorporeal blood circuit of the dialysismachine during the experimental tests to which the graph of FIG. 2relates.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the number 1 indicates the whole of a dialysis machineconnected to a patient P. The machine 1 comprises an extracorporealblood circuit 2, a dialysate circuit 3 and a filter 4, which comprises ablood compartment 5 and a dialysate compartment 6 separated by asemi-permeable membrane 7.

The extracorporeal blood circuit 2 comprises an arterial branch 8, inwhich is located a peristaltic pump 9 providing a flow Qb of blood, anda venous branch 10, in which is located a dropper 11. The arterialbranch 8 has one end connected to the blood compartment 5 and one endprovided with an arterial needle 12, which, when in use, is insertedinto a fistula (not shown) in the patient P to collect the blood fromthe cardiovascular system of the patient P, while the branch 10 has oneend connected to the blood compartment 5 and an opposite end providedwith a venous needle 13, which, when in use, is inserted into theaforesaid fistula (not shown) to return the treated blood to thecardiovascular system of the patient P. The branches 8 and 10 are tubesmade from plastic material, generally PVC, and are used, respectively,to supply the blood to be treated to the compartment 5 and to introducethe treated blood leaving the compartment 5 into the cardiovascularsystem. The filter 4 and the dropper 11 are also, made from plasticmaterial, generally PVC.

The machine 1 also comprises a device 14 for detecting the detachment ofthe venous needle 13. The operation of the device 14 is based onexperimental measurements carried out by the applicant. The graphs inFIGS. 2 to 4 relate to experimental measurements made by the applicantand show a transitory time interval _t1 in which the blood flow Qb is,made to vary; two time intervals _t2 and _t3 in which the blood flow Qbis kept essentially constant; and a time interval _t4 in which the bloodflow is reduced to zero, or in other words the dialysis treatment isinterrupted. The measurements which were made indicated that, duringdialysis treatment in standard conditions, in other words with aconstant blood flow Qb, the variations of the arterial pressure Pa andvenous pressure Pv found in the arterial branch 8 and in the venousbranch 10 respectively are essentially of opposite sign. In other words,a decrease in arterial pressure Pa in the arterial branch 8 generallycorresponds to an increase in the pressure Pv in the venous branch 10,and vice versa. The applicant has found that, when the venous needle 13is detached in the interval _t3, both the arterial pressure Pa in thearterial branch 8 and the venous pressure Pv in the venous branch 10decrease significantly when the blood flow Qb is constant.

In order to determine the variations of arterial pressure Pa and venouspressure Pv indicating the detachment of the venous needle 13, thedevice 14 comprises a control unit 15, a pressure detector 16 located inthe arterial branch 8 up-line from the peristaltic pump 9, a pressuredetector 17 located in the venous branch 10 near the dropper 11, and anencoder 18 for measuring the speed of revolution of the rotor of theperistaltic pump 9. The control unit 15 receives a signal Sr from theencoder 18, correlated with the speed of revolution of the rotor of theperistaltic pump 9, and two signals Spa and SPv, correlated with thearterial pressure Pa and venous pressure Pv, from the two detectors 16and 17 respectively.

The unit 15 can process the signals Sr, Spa and SPv, and comprises threederivation units 19 p and 19 r, which derive the signals Spa, SPv and Srwith respect to time and supply corresponding signals dSpa, dSPv anddSr, two rectification units 20 for rectifying the derived signals dSPaand dSPv and for supplying two signals RSPa and RSPv, a comparison unit21, which receives the signal dSR and compares it with an interval I ofacceptability stored in a store 22 and emits a digital signal SD1 at theoutput as a function of the comparison with the interval I ofacceptability. The unit 15 comprises an adder unit 23, which adds therectified signals RSPa and RSPv, and emits a signal SA equal to the sumof the rectified signals RSPa and RSPv, a comparison unit 24, whichreceives the output signal SA from the adder unit 23, compares thesignal SA with a threshold value TV contained in a store 25, and emits adigital signal SD2. The unit 15 also comprises two conversion units 26for converting the signals RSPa and RSPv into digital signals SD3 andSD4 respectively, and a logical operator unit 27 which receives thedigital signals SD1, SD2, SD3 and SD4, compares them with a truth table,and emits a control signal SC.

The dialysis machine 1 comprises a control unit 28, which is connectedto the control unit 15 of the device 14 for receiving the control signalSC, and is also connected to the peristaltic pump 9. The unit 28comprises an acoustic warning device 29 and a visual indicator 30 toalert the operator and user to the detachment of the venous needle 13.

In use, the dialysis machine 1 is connected to a patient P as shownschematically in FIG. 1. During a dialysis treatment in standardconditions, the blood flow Qb is kept essentially constant as shown inthe graph of FIG. 2 in the intervals _t2 and _t3. In FIG. 2, the bloodflow Qb varies in an initial time interval _t1, and is then keptconstant in successive time intervals _t2 and _t3, until it is broughtto zero in the interval _t4 following the detachment of the venousneedle 13.

The graphs of FIGS. 3 and 4, which have a time scale identical to thetime scale, of the graph of FIG. 2, show the generally differing trendsof the variations of the venous pressure Pv and arterial pressure Pawith a constant flow Qb in the time interval _t2, and the uniformdecrease in the venous pressure Pv and arterial pressure Pa in the timeinterval _t3, during which the venous needle 13 has become detached andthe peristaltic pump 9 has not yet been stopped. The peristaltic pump 9is then stopped and the dialysis treatment is interrupted to preventloss of blood from the patient P.

With reference to FIG. 1, during the dialysis treatment the speed ofrevolution of the peristaltic pump 9 and the arterial pressure Pa andvenous pressure Pv are constantly measured by the encoder 18 and by thedetectors 16 and 17. The speed of revolution of the rotor of theperistaltic pump 9 is correlated with the flow Qb, and this measurementis equivalent for practical purposes to a measurement of the flow Qb,once the characteristics of the pump 9 and the dynamic conditions of theextracorporeal circuit 2 are known. The pressure signals SPa and SPbundergo derivation in the corresponding derivation units 19 p, and thederived signals dSPa and dSPv are rectified in the correspondingrectification units 20. In the rectification unit 20, each of thesignals RSPa and RSPv takes a value of zero when the corresponding inputsignal dSPa or dSPv is positive (increasing pressure) and takes apositive value, proportional to the input signal dSPa or dSPv, when thecorresponding input signal dSPa or dSPv is negative (falling pressure).The derived signal dSr is compared with the interval I of acceptabilityin the comparison unit 21, which supplies a digital signal SD1 which isequal to one when the derived signal dSr falls within the acceptabilityinterval (constant flow Qb) and is equal to zero when the derived signaldSr falls outside the acceptability interval (variable flow Qb). Inpractice, the signal SD1 is equal to one when the flow Qb is essentiallyconstant, and equal to zero when the flow Qb is significantly varied.

The adder unit 23 adds the rectified signals RSPa and RSPv and comparesthe sum SA with the threshold value TV, and supplies a digital signalSD2 which is equal to one when the sum SA is greater than the thresholdvalue TV, and a digital signal SD2 which is equal to zero when the sumSA is smaller than the threshold value TV.

The values of the digital signals SD3 and SD4 are equal to one when thecorresponding rectified signals RSPa and RSPv are greater than zero, andequal to zero when the corresponding rectified signals RSPa and RSPv areequal to zero.

The logical operator unit 27 receives the signals SD1, SD2, SD3 and SD4and compares these signals with a truth table, and supplies at itsoutput a digital signal SC which is equal to one when all the signalsSD1, SD2, SD3 and SD4 are equal to one, while the signal SC takes avalue equal to zero when only one of the signals SD1, SD2, SD3 and SD4is equal to zero. From the practical point of view, with reference toFIGS. 2 to 4 and to the time interval _t2, the flow Qb is constant andthe signal SD1 is equal to one, the rectified signals SD3 and SD4 willboth be equal to zero or at least one will be equal to zero. Because ofthe slight variations of the pressures Pa and Pv in the interval _t2,the sum SA is less than the threshold value TV, which is selectedexperimentally to discriminate negligible variations of the arterialpressures Pa and venous pressures Pv from significant variations.Consequently, the signal SC is equal to zero.

With reference to the interval _t3, the abrupt decrease of the arterialpressures Pa and venous pressures Pv causes both the signals RSPa andRSPv to be positive and their sum SA to be greater than the thresholdvalue TV. This causes SD2, SD3 and SD4 to be equal to one. The constancyof the flow Qb also causes the signal SD1 to be equal to one, andtherefore the signal SC will also be equal to one, which is the valuethat indicates that the venous needle has become detached.

The signal SD2 provides a measurement of the degree of the decrease ofthe arterial pressures Pa and venous pressures Pv and the thresholdvalue TV makes it possible to discriminate the very slight decreases inthe aforesaid pressures, which may occur from time to time even withoutthe detachment of the venous needle, from the significant decreaseswhich occur when the venous needle 13 actually becomes detached.

The signal SC is transmitted to the unit 28, which emits an alarm signalAS to stop the peristaltic pump 9 and the dialysis treatment when SC isequal to one.

As an alternative to the stopping of the peristaltic pump 9, or inaddition to it, the alarm signal AS can trigger the activation of theacoustic warning device 29 and/or the visual indicator 30.

1. A method of detecting the detachment of a venous needle from apatient during an extracorporeal blood treatment in a dialysis machinehaving an extracorporeal blood circuit comprising an arterial branch anda venous branch, the method comprising the steps of: measuring anarterial pressure in the arterial branch and a venous pressure in thevenous branch; determining whether or not decreases of the arterialpressures and venous pressures are occurring during the dialysistreatment in normal conditions; measuring a blood flow along theextracorporeal blood circuit; determining whether the blood flow isessentially constant during a time interval in which said decreases ofarterial pressure and venous pressure have occurred; and emitting analarm signal when all of the following conditions are present duringsaid time interval: a decrease in the arterial pressure; a decrease inthe venous pressure; and the blood flow along the extracorporeal bloodcircuit is essentially constant.
 2. The method according to claim 1,wherein the value of said decreases of arterial pressure and venouspressure is evaluated.
 3. The method according to claim 2, wherein asignal correlated with the sum of the decreases of arterial pressure andvenous pressure is compared with an experimentally determined thresholdvalue.
 4. The method according to claim 3, wherein an alarm signal isemitted when the additional following condition is present during saidtime interval: the signal correlated with the sum of the decreases ofarterial pressure and venous pressure is greater than said thresholdvalue.
 5. The method according to claim 4, wherein said blood flow isstopped in said extracorporeal circuit by means of said alarm signal, tointerrupt the dialysis treatment.
 6. The method according to claim 4,wherein an acoustic warning device is activated by means of said alarmsignal.
 7. The method according to claim 4, wherein a visual indicatoris activated by means of said alarm signal.
 8. The method according toclaim 1, wherein measuring a blood flow along the extracorporeal bloodcircuit comprises measuring a speed of a pump providing said blood flow.9. The method according to claim 8, wherein the pump providing saidblood flow is a peristaltic pump located in said arterial branch. 10.The method according to claim 8, wherein measuring a speed of a pumpcomprises measuring a speed of revolution of a rotor of the pump.
 11. Adevice for detecting the detachment of a venous needle from a patientduring an extracorporeal blood treatment in a dialysis machine having anextracorporeal blood circuit comprising an arterial branch and a venousbranch, the device comprising: pressure detectors for measuring anarterial pressure in the arterial branch and a venous pressure in thevenous branch; means for measuring a blood flow along the extracorporealblood circuit; and a control unit comprising: means for determiningwhether or not decreases of arterial pressure and venous pressure areoccurring during the dialysis treatment in normal conditions; means fordetermining whether the blood flow is essentially constant during a timeinterval in which said decreases of arterial pressure and venouspressure have occurred; and means for emitting an alarm signal when allof the following conditions are present during said time interval: adecrease in the arterial pressure; a decrease in the venous pressure;and the blood flow along the extracorporeal blood circuit is essentiallyconstant.
 12. The device according to claim 11, wherein said emittingmeans comprise means for emitting an alarm signal when the additionalfollowing condition is present during said time interval: a signalcorrelated with the sum of the decreases of arterial pressure and venouspressure is greater than a threshold value.
 13. The device according toclaim 11, wherein said determining means comprise a derivation unit foremitting a signal correlated with the derivative of the blood flow and afirst comparison unit for determining whether the signal correlated withthe derivative of the blood flow falls within an interval ofacceptability.
 14. The device according to claim 11, wherein saidcontrol unit comprises means for discriminating the value of thedecreases of the arterial pressure and venous pressure.
 15. The deviceaccording to claim 13, wherein said control unit comprises a logicaloperator unit which can emit a control signal as a function of thedecreases of arterial pressure and venous pressure, and an output signalfrom the first comparison unit.
 16. The device according to claim 11,wherein said means for measuring a blood flow comprise means formeasuring a speed of a pump providing said blood flow.
 17. The deviceaccording to claim 16, wherein the pump providing said blood flow is aperistaltic pump located in said arterial branch.
 18. The deviceaccording to claim 16, wherein the means for measuring a speed of a pumpcomprise an encoder for measuring a speed of revolution of a rotor ofthe pump.
 19. A dialysis machine comprising: an extracorporeal bloodcircuit comprising an arterial branch and a venous branch; and a devicefor detecting the detachment of a venous needle from a patient during anextracorporeal blood treatment, the device comprising: pressuredetectors for measuring an arterial pressure in the arterial branch anda venous pressure in the venous branch; means for measuring a blood flowalong the extracorporeal blood circuit; and a control unit comprising:means for determining whether or not decreases of arterial pressures andvenous pressures are occurring during the dialysis treatment in normalconditions; means for determining whether the blood flow is essentiallyconstant during a time interval in which said decreases of arterialpressure and venous pressure have occurred; and means for emitting analarm signal when all of the following conditions are present duringsaid time interval: a decrease in the arterial pressure; a decrease inthe venous pressure; and the blood flow along the extracorporeal bloodcircuit is essentially constant.
 20. The dialysis machine according toclaim 19, further comprising a peristaltic pump located in said arterialbranch, and a dropper located in said venous branch.
 21. The dialysismachine according to claim 20, wherein said pressure detectors comprisea first detector located in the arterial branch up-line from theperistaltic pump, and a second detector located in the venous branchnear the dropper.
 22. The dialysis machine according to claim 19,further comprising an additional control unit comprising means forreceiving a control signal from said control unit and emitting an alarmsignal as a function of the control signal, said additional control unitcomprising a warning device which can be activated by means of saidalarm signal.
 23. The dialysis machine according to claim 19, furthercomprising an additional control unit which can receive a control signalfrom said control unit and emit an alarm signal as a function of thecontrol signal, said additional control unit comprising a visualindicator which can be activated by means of said alarm signal.
 24. Thedialysis machine according to claim 19, further comprising an additionalcontrol unit comprising means for receiving a control signal from saidcontrol unit and emitting an alarm signal as a function of the controlsignal, said alarm signal being emitted when all the followingconditions are present during said time interval: a decrease in arterialpressure; a decrease in venous pressure; a signal correlated with thesum of the decreases of arterial pressure and venous pressure is greaterthan a threshold value; and the blood flow is essentially constant. 25.The dialysis machine according to claim 19, wherein the means formeasuring a blood flow along the extracorporeal blood circuit comprisemeans for measuring a speed of a pump providing said blood flow.
 26. Thedialysis machine according to claim 25, wherein the pump providing saidblood flow is a peristaltic pump located in said arterial branch. 27.The dialysis machine according to claim 25, wherein the means formeasuring a speed of a pump comprise an encoder for measuring a speed ofrevolution of a rotor of the pump.